The present invention relates generally to optical recording systems, and more particularly, to maintaining the write/read beam position on the data area of an optical disk very accurately.
Due to the high track density of optical recording it is imperative to maintain the write/read beam position on the data area of an optical disk very accurately. This is done by means of a servo mechanism. Currently there are many methods for obtaining radial position servo error information, and two popular proposed media standards are discussed below.
One standard is based on a continuous groove formed in the optical disk. The recording areas (lands) are separated by shallow grooves. The lands and grooves are typically circular or spiral in shape. Typical center to center spacing between the grooves is 1.6 microns, and the groove width is 0.7 microns and its depth is 0.05 microns. The write/read beam falls predominantly on the lands, but some of its energy impinges on neighboring grooves. Beam motion off the center of the land results in changes in the intensity distribution of the reflected beam. This asymmetry is detected using a split photodetector, for example. The difference in the output of the two halves of the split photodetector produces a tracking error signal. The tracking error signal derived in this manner is usually called a "push-pull" error signal.
The other standard is based on sampling information features prerecorded on the optical disk. Usually this information is prerecorded in an unerasable fashion during substrate fabrication, and consists of pairs of bits (pits or bumps on the disk surface) one after another symmetrically offset from the centerline of each of the lands. The tracking error signal is derived by comparing the signal amplitude from the members of a two-bit pair and holding it until the next pair comes under the beam. In the case of erasable media, the servo sampling information is recorded in the same way as other data, for example. This sampling servo method is implemented having the servo information processed through the data channel between every 16 bytes of user data.
Both of the above methods have their advantages and disadvantages. Specifically, the sampling method is much less sensitive to disk tilt and detector mispositioning errors. On the other hand the sampling method imposes overhead on the data channel, and introduces severe timing problems. The continuous groove tracking method has the major disadvantage of introducing offsets due to disk tilt, detector misalignment, detector and amplifier offsets, beam decentration and lens decentration.
Accordingly, it is an objective of the present invention to provide an optical recording system that maintains the write/read beam position on the data area of an optical disk very accurately. Another objective of the invention is the provision of an optical recording system that controls the write/read beam position accurately without adding additional information to the disk. A further objective of the present invention is to provide an optical recording system that controls the write/read beam position accurately without imposing overhead on the data channel.
Yet another objective of the invention is the provision of a recording system that controls the write/read beam position in such a manner to provide electronic compensation of tracking offsets of tracking level shifts. It is a further objective of the present invention to provide for an offset compensation technique that compensates for effects of disk tilt, detector misalignment detector and amplifier offsets, and beam decentration due to fine tracking.